Metal oxide structure containing Titanium oxide and production method and use thereof

ABSTRACT

A dye sensitized solar cell comprising, as the dye electrode, a titanium oxide structure having an optical band gap (hereinafter referred to as “BG”) of 2.7 to 3.1 eV as calculated from absorbance measured by an integrating sphere-type sptetrophotometer, or a metal oxide structure obtained by dry-mixing a plurality of metal oxide powder particles differing in the particle size or a metal oxide dispersion thereof, wherein assuming that the BG of raw material metal oxide is BGO and the BG of metal oxide after the dry mixing is BG1, the (BG0-BG1) is from 0.01 to 0.45 eV, and a production method thereof are provided.

CROSS REFERENCE TO RELATED APPLICATION

This application is an application filed under 35 U.S.C. §111(a)claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date ofthe Provisional Application No. 60/416,919 filed on Oct. 9, 2002, andthe filing date of the Provisional Application No. 60/489,109 filed onJul. 23, 2003, pursuant to 35 U.S.C. §111(b).

FIELD OF THE INVENTION

The present invention relates to a method for producing a titaniumoxide-containing metal oxide structure, which is suitably used for solarcells and the like.

DESCRIPTION OF RELATED ART

At present, silicon-type solar cells are predominating as the solar cellbut in view of use of harmful raw materials, high production cost andthe like, studies are being made to develop new-style solar cell. One ofsolar cells thus developed is a dye sensitized solar cell and sinceGraetzel et al. of EPFL-Lausanne reported it in 1991 (see, for example,Japanese Unexamined Patent Publication (Kokai) No. 2001-283942), thiscell is being studied and developed as an alternative to thesilicon-type cell. The dye sensitized solar cell in general has astructure shown in FIG. 1 and comprises three portions of a dyeelectrode 6, an electrolytic layer 4 and a counter electrode 7. Here,the dye electrode 6 indicates an electrode comprising an electrodesubstrate (e.g., electrically conducting glass) having formed thereon ametal oxide layer (e.g.a titanium oxide, combined with a sensitizingdye, and the counter electrode 7 indicates an electrode comprising anelectrode substrate (e.g., electrically conducting glass) having formedthereon a catalyst layer (e.g., platinum, graphite). The electrolyticlayer 4 is a solution having dissolved therein an electrolyte and thisis a portion sandwiched by the dye electrode and the counter electrode.The electrode substrate as used herein indicates an electrode substrate(e.g., glass, organic polymer) on which FTO, ITO or the like is coatedand dried.

The mechanism of photoelectric conversion is described as follows.

First, the sensitizing dye absorbs light to generate an electron and ahole. The electron generated reaches through the metal oxide layer theelectrode substrate and is taken outside. On the other hand, the holegenerated is transferred through the electrolytic layer to the counterelectrode and combines with an electron supplied through the electrodesubstrate.

As an index showing the characteristics of the dye sensitized solarcell, a photoelectric conversion efficiency represented by the followingformula is known.η(%)=Jsc×Voc×FF/incident light energy×100wherein η is a photoelectric conversion efficiency, Jsc is ashort-circuit current density [mA/cm²], Voc is an open voltage [V], FFis a fill factor [−] and the incident light energy is an incident lightenergy [mw/cm²] per unit area. The photoelectric conversion efficiency ηdepends on the performance of dye electrode. The factor of enhancing theperformance of dye electrode includes increasing the surface area perunit of metal oxide to increase the amount of sensitizing dye supportedand thereby increase the amount of electron generated, and increasingnecking of metal oxide particles to attain smooth electron transfer. The“necking of particles” as used herein means a structure as shown in FIG.2 and this is distinguished from the point contact structure ofparticles with each other.

These factors are described below by using titanium oxide as an exampleof the metal oxide.

As the method for increasing the surface area per unit of metal oxide, amethod of using titanium oxide having a pore inner diameter of 3 to 10nm is described (see, for example, Japanese Unexamined PatentPublication (Kokai) No. 2001-283942). According to this method, a solprepared from titanium tetrachloride is heated and dried to obtaintitanium oxide particles. However, the liquid-phase process titaniumoxide obtained by the hydrolysis of titanium tetrachloride or the likeundergoes a low heat history at the synthesis and the necking structureis disadvantageously less formed.

For increasing the necking of metal oxide particles, a method of forminga titanium oxide layer on a substrate such as electrically conductingglass and then treating it with titanium tetrachloride has been proposed(see, for example, C. J. Barbe et al., J. Am. Ceram. Soc., 80, 3157(1997)). Here, the titanium tetrachloride has an activity of reactingwith a titanium oxide particle to generate new bonding and thereby neckthe particles with each other. As such, the titanium tetrachloridetreatment increases necking, however, this treatment has a problem inthat crystallinity on the surface of titanium oxide particle decreasesor lattice defects are generated. If the crystallinity is low or latticedefects are present, the conduction band energy level of titanium oxidedecreases and when a solar cell is produced, the open voltage lowers andthe photoelectric conversion efficiency decreases.

As another method for promoting the electron transfer, a method ofmixing particle groups different in the particle size and therebyenhancing the filling density of particles is known. For example, 2method of using semiconductor particle groups having a plurality ofpeaks in the particle size distribution has been proposed (see, forexample, Japanese Unexamined Patent Publication (Kokai) No.2001-357899). However, since a plurality of particle groups are merelymixed, only the point contact among particles is increased and theelectron transfer efficiency is lower than in the case of neckingstructure.

The present invention has been made to solve the above-describedproblems and an object of the present invention is to provide aproduction method of a titanium oxide structure and a metal oxidesuitable for solar cells, which ensures a large adsorbed amount ofsensitizing dye, smooth electron transfer and high photoelectronconversion efficiency.

SUMMARY OF THE INVENTION

As a result of extensive investigations to solve those problems, thepresent inventors have found out a method capable of producing a metaloxide structure where the surface area per unit mass is large andparticles are necked with each other. The above-described object can beattained by this finding.

More specifically, the object of the present invention can be attainedby developing:

[1] a titanium oxide structure having an optical band gap (hereinafterreferred to as “BG”) of 2.7 to 3.1 eV as calculated from absorbancemeasured by an integrating sphere-type spectrophotometer and having atap density of 0.15 to 0.45 g/cm³;

[2] a metal oxide structure obtained by dry-mixing a plurality of metaloxide powder particles differing in the particle size, wherein assumingthat an optical band gap (hereinafter referred to as “BG”) of rawmaterial metal oxide is BG0 and the BG of metal oxide after the drymixing is BG1, the (BG0-BG1) is from 0.01 to 0.45 eV;

[3] a method for producing a metal oxide structure, comprising drymixing a metal oxide, wherein assuming that an optical band gap(hereinafter referred to as “BG”) of raw material metal oxide is BG0 andthe BG of metal oxide after the dry mixing is BG1, the mixing isperformed to give a (BG0-BG1) of 0.01 to 0.45 eV;

[4] the method for producing a metal oxide structure as described in [3]above, wherein the dry mixing is at least one method selected from aball mill, a high-speed rotary grinder, a stirring mill and a jetgrinder;

[5] the method for producing a metal oxide structure as described in [3]above, wherein the dry mixing is performed by a ball mill and assumingthat the total mass of powder particles mixed is wp (g), the mass ofmedium is wm (g), the inner diameter of ball mill container is d (m),the rotation number is n (rpm) and the mixing time is t (minute), theenergy constant k1 at the dry mixing represented by the followingrelationship:i k1=wm/wp×d×n×tis from 3,000 to 250,000;

[6] the method for producing a metal oxide structure as described in [5]above, wherein the energy constant k1 is from 10,000 to 150,000;

[7] the method for producing a metal oxide structure as described in [5]above, wherein the energy constant k1 is from 10,000 to 50,000;

[8] the method for producing a metal oxide structure as described in anyone of [3] to [7] above, wherein the raw material metal oxide comprisesa metal oxide powder having an average primary particle size of 100 to500 nm (hereinafter referred to as Particle Group A) and a metal oxidepowder having an average primary particle size of 10 to 40 nm(hereinafter referred to as Particle Group B), the converted from thespecific surface area determined by the BET method particle sizes beingconverted from the specific surface area determined by the BET method.

[9] the method for producing a metal oxide structure as described in [8]above, wherein Particle Group B is a mixture of a metal oxide powderhaving an average primary particle size of 20 to 40 nm (hereinafterreferred to as Particle Group C) and a metal oxide powder having anaverage primary particle size of 10 to 20 nm (hereinafter referred to asParticle Group D), the particle sizes being converted from the specificsurface area determined by the BET method;

[10] the method for producing a metal oxide structure as described in[8] or [9] above, wherein the average specific surface area of ParticleGroup B is from 60 to 110 m²/g;

[11] the method for producing a metal oxide structure as described inany one of [8] to [10] above, wherein at least one of Particle Groups Ato D is a metal oxide synthesized by a gas phase process;

[12] the method for producing a metal oxide structure as described inany one of [3] to [11] above, wherein the tap density is from 0.15 to1.0 g/cm³;

[13] the method for producing a metal oxide structure as described inany one of [3] to [12] above, wherein the metal oxide is titanium oxide;

[14] the method for producing a metal oxide structure as described inany one of [3] to [12] above, wherein the metal oxide is a mixture oftitanium oxide and at least one metal oxide selected from zinc oxide,niobium oxide, tantalum oxide, zirconium oxide, tin oxide and tungstenoxide;

[15] the method for producing a metal oxide structure as described in[14] above, wherein the content of titanium oxide contained in the metaloxide mixture is 10 mass % or more;

[16] a method for producing a metal oxide dispersion, comprising addinga dispersion medium to the titanium oxide structure described in [1]above, the metal oxide structure described in [2] above or a metal oxidestructure obtained by the production method described in any one of [3]to [15] above, and wet-mixing these by a ball mill, wherein assumingthat the total mass of powder particles mixed is wp (g), the mass ofmedium is wm (g), the inner diameter of ball mill container is d (m),the rotation number is n (rpm) and the mixing time is t (minute), theenergy constant k2 at the wet mixing represented by the followingrelationship:k2=wm/wp×d×n×tand the energy constant k1 at the dry mixing satisfy the followingrelationship:k2≦k1;

[17] the method for producing a metal oxide dispersion as described in[16] above, wherein the energy constant k2 at the wet mixing and theenergy constant k1 at the dry mixing satisfy the following relationship:8.0×k1≧k2≧1.5×k1;

[18] the method for producing a metal oxide dispersion as described in[16] above, wherein the energy constant k2 at the wet mixing and theenergy constant k1 at the dry mixing satisfy the following relationship:5.0×k1≧k2≧2.5×k1;

[19] a titanium oxide-containing metal oxide dispersion obtained by theproduction method described in any one of [16] to [18] above;

[20] a composition comprising the titanium oxide structure described in[1] above, the metal oxide structure described in [2] above, a metaloxide structure obtained by the production method described in any oneof [3] to [15] above, or the titanium oxide-containing metal oxidedispersing element described in [19] above;

[21] a thin film comprising the titanium oxide structure described in[1] above, the metal oxide structure described in [2] above, a metaloxide structure obtained by the production method described in any oneof [3] to [15] above, or the titanium oxide-containing metal oxidedispersion described in [19] above;

[22] the metal oxide structure-containing thin film as described in [21]above, wherein the film has a thickness of from 1 to 40 μm;

[23] a method for producing a dye sensitized solar cell, comprisingincluding the metal oxide structure obtained by the production methoddescribed in any one of [3] to [15] above as a dye sensitized electrode;

[24] a method for producing a dye sensitized solar cell, comprisingincluding the metal oxide structure obtained by the production methoddescribed in any one of [3] to [15] above and the metal oxide dispersiondescribed in any one of [17] to [19] above as a dye sensitizedelectrode;

[25] a dye sensitized solar cell produced by the production methoddescribed in [23] or [24] above;

[26] a dye sensitized solar cell equipped with a dye electrodecomprising, as a constituent member, the metal oxidestructure-containing thin film described in [22] above;

[27] a dye sensitized solar cell, wherein an optical band gap(hereinafter referred to as “BG”) of titanium oxide after removing thedye from the dye electrode is 2.7 to 3.1 eV;

[28] an article having a power-generating function, equipped with thedye sensitized solar cell described in any one of [25] to [27] above;

[29] an article having a light-emitting function, equipped with the dyesensitized solar cell described in any one of [25] to [27] above;

[30] an article having a heat-generating function, equipped with the dyesensitized solar cell described in any one of [25] to [27] above;

[31] an article having a sound-generating function, equipped with thedye sensitized solar cell described in any one of [25] to [27] above;and

[32] an article having a moving function, equipped with the dyesensitized solar cell described in any one of [25] to [27] above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section showing an outline of the constitution of adye sensitized solar cell.

FIG. 2 is an electron micrograph showing the necking state of titaniumoxide particles.

FIG. 3 is an absorbance pattern for determining the absorption edgewavelength.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

The metal oxide structure of the present invention is a structurecontaining metal oxide particle microstructure. The metal oxidestructure of the present invention is a structure characterized bycontaining necking structure (partial area bonding or contact structure)of particles.

The metal oxide structure of the present invention is obtained bydry-mixing a metal oxide powder having an average primary particle sizeof 100 to 500 nm (hereinafter referred to as Particle Group A) and ametal oxide powder having an average primary particle size of 10 to 40nm (hereinafter referred to as Particle Group B), the particle sizesbeing converted from the specific surface area determined by the BETmethod. The dry mixing as used herein means a method of mixing thepowder particles without using a dispersion medium such as water or anorganic solvent. In the wet mixing using a dispersion medium, the energygenerated due to collision, friction or the like diffuses not only intothe particles but also into the dispersion medium and therefore, amechanochemical reaction is difficult to occur. The important point isto allow a mechanochemical reaction to proceed by dry mixing and attainnecking of particles with each other.

In the state where Particle Group A and Particle Group B are merelymixed, the particles make only point contact with each other andtherefore, the electron transfer efficiency is low as compared with theparticle structure having a necking structure (partial plane contactstructure) as shown in FIG. 2. For attaining smooth electron transfer,it is important to have a necking structure of particles with eachother.

A so-called liquid-phase process metal oxide obtained by the wethydrolysis of a halogenated metal or the like undergoes a low heathistory at the synthesis. In this state, the necking structure is lessformed and therefore, the electron transfer efficiency is low. On theother hand, a metal oxide particle group obtained by a so-calledvapor-phase process of reacting a halogenated metal or the like with anoxidizing gas such as oxygen at a high temperature is subject to a highheat history at the synthesis and therefore, this metal oxide particlegroup has a high crystallinity and at the same time, has a necking bond,as a result, a structure facilitating in the electron transfer andadvantageous to the diffusion of an electrolytic solution is obtained.This production method by a vapor-phase process is not particularlylimited but in the case of titanium oxide, its synthesis can beperformed, for example, by the method of Patent Documents 3, 4 or 5.

In the production method of the present invention, at least one particlegroup of the metal oxides constituting the metal oxide structure ispreferably obtained by a vapor-phase process. As described above, in thecase of a metal oxide obtained by a vapor-phase process, the particlesthemselves form a necking structure with each other to some extent andin such a structure, the electron transfer is somewhat facilitated. Theelectron transfer may be more promoted by further increasing the numberof particles forming a necking structure. In this meaning, theemployment of a mechanochemical reaction by dry mixing is moreeffective. According to the vapor-phase process, a powder having arelatively narrow primary particle size distribution can be obtained ascompared with other production methods and therefore, when this powderis used as Particle Group A or B, a primary particle size distributionpreferred as the metal oxide structure of the present invention iseasily obtained.

In the production method of the present invention, Particle Group Aconstituting the metal oxide structure mainly acts on light intrudedinto a solar cell to scatter the light inside the cell and therebyincrease the light absorption efficiency. In a dye sensitized solarcell, light over the region from ultraviolet to near infrared isabsorbed to generate an electron and therefore, when light over theregion from ultraviolet to near infrared is scattered inside the solarcell, the probability of light absorption by a sensitizing dye or thelike increases. In general, it is known that maximum light scattering isobtained when the particle size is about a half of the light wavelength,and as the particle size more deviates therefrom, the light scatteringis also lessened (see, Non-Patent Document 3). In order to scatter lightover the region from ultraviolet to near infrared, the average primaryparticle size is preferably from 100 to 500 nm. The particle size canalso be selected according to the wavelength of light intended toscatter.

Particle Group B has a role of supporting a sensitizing dye andtransmitting an electron generated by the sensitizing dye. Thesensitizing dye is supported on the metal oxide by chemical bonding withthe surface hydroxyl group or metal atom (hereinafter referred to as a“dye bonding portions) of metal oxide and transfers an electron to themetal oxide through the bond. Therefore, as the amount of sensitizingdye bonded to the metal oxide increases, the number of electronstransferred increases. The metal oxide particle, for example, titaniumoxide particle is known to have from 9 to 14 surface hydroxyl groups/nm²on the surface thereof (see, Manabu KIYONO, Sanka Titan (TitaniumOxide), supra, pp. 54-55) and as the titanium oxide has a largerspecific surface area, the dye bonding portion more increases. Thespecific surface area of Particle Group B suitable for solar cells isfrom about 40 m²/g to about 150 m²/g, preferably from about 60 m²/g toabout 110 m²/g.

In the case of titanium oxide, when the specific surface area isconverted into an average primary particle size, this is from about 10nm to about 40 nm, preferably from about 13 nm to about 25 nm. If theparticle group has an average primary particle size of less than about10 nm, the crystallinity is generally low and smooth electron transferis not attained, therefore, this particle group is not suitable for usein a solar cell. This low crystallinity is attributable to the heathistory suppressed low at the synthesis so as to prevent the growth ofparticle. If the particle group has an average primary particle sizeexceeding about 40 nm, the specific surface area is small and the amountof dye adsorbed is insufficient. The blending ratio of Particle Group Aand Particle Group B is A/B=from 5/95 to 30/70 (by mass), preferablyA/B=from 10/90 to 20/80 (by mass).

In order to attain smooth electron transfer, the particles preferablyform a necking structure with each other and at the same time, aredensely filled. For elevating the filling density, the method ofcombining particle groups differing in the average particle size issimple and easy. Particle Group B classified into an ultrafine particlemay be used as it is (single particle group) but the filling density islow in many cases, therefore, particle groups differing in the particlesize are preferably combined to elevate the filling density, wherebygood results can be obtained. The average primary particle size of eachparticle group used as the base for the combination is preferablyselected from the particle size range suitable for Particle Group B. Acombination of Particle Group C having an average primary particle sizeCf 20 to 40 nm and Particle Group D having an average primary particlesize of 10 to 20 nm is preferred. The blending ratio of Particle Group Cand Particle Group D is C/D=from 10/90 to 80/20 (by mass), preferablyC/D=from 15/85 to 75/25 (by mass).

The chemical compositions of Particle Groups A, B and C may be the sameor different from each other.

One of indices showing the filling state of particles is the tapdensity. As the filling density elevates, the value increases. The tapdensity is measured by the following method.

A powder tester such as Type PT-D manufactured by Hosokawa MicronCorporation is used. A sample is filled in a 100 cm³-volume cup with anauxiliary cup and subjected to tapping 100 times by the powder tester.After the auxiliary cup is removed, the sample is exactly filled to 100cm³ and the mass (g) of the sample is measured. The tap density isobtained by dividing the mass (g) of the powder particles by 100.

In the production method of the present invention, the metal oxidestructure obtained preferably has a tap density of 0.15 to 1.0 g/cm³. Ifthe tap density is less than 0.15 g/cm³, this reveals insufficientfilling density, whereas if the tap density exceeds 1.0 g/cm³, the metaloxide structure is difficultly dispersed when used as a dispersion. Themetal oxide structure dispersion in a poor dispersed state is reduced inthe void portion of the metal oxide structure and when a dye sensitizedsolar cell is produced, the electrolyte scarcely diffuses into the metaloxide layer to cause shortage of electrolyte inside the metal oxidelayer and the shortage of electrolyte gives rise to failure in thesmooth transfer of electric charge.

In the dry mixing, for example, a ball mill, a high-speed rotarygrinder, a stirring mill or a jet grinder is used. As long as an energyof causing a mechanochemical reaction in the particle groups isimparted, any mill or grinder may be used but the equipment used ispreferably formed of a material difficult to contaminate. In thefollowing, an example of using a rolling ball mill among ball mills isdescribed.

The rolling ball mill is a most general-purpose mixing and grindingmethod and this method utilizes the collision, frictional action or thelike between the powder particles and the mediums in the container,which occurs resulting from rolling of a cylindrical container. In thiscase, the energy constant k which has been proposed as an index for theunified evaluation of the mixing and grinding effect by a rolling ballmill (see, Non-Patent document 4) is represented by the followingformula:k=wm/wp×d×n×twherein wp represents a total mass (g) of powder particles mixed, wmrepresents a mass (g) of medium, d represents an inner diameter (m) ofball mill container, n represents a rotation number (rpm) and trepresents a mixing time (minutes).

As the energy constant elevates, the collision or friction energyimposed on the powder particle increases and the mechanochemicalreaction more easily proceeds, as a result, aggregation takes place morevigorously.

In the method for producing a metal oxide structure of the presentinvention, assuming that the energy constant at dry mixing is k, k1 ispreferably from 3,000 to 250,000. If the energy constant k1 is less thanthe lower limit, the mechanochemical reaction proceeds insufficientlyand the particles are less bonded with each other, whereas if the energyconstant k1 exceeds the upper limit, the mechanochemical reactionproceeds but when the metal oxide structure is used in a dispersion itis difficult to be dispersed and the void portion of the obtained metaloxide structure is reduced. The reduction in the void portion adverselyaffects the diffusion of electrolyte when a dye sensitized solar cell isproduced, giving rise to decrease in the performance of the solar cell.Furthermore, the excessive mechanochemical reaction extremely lowers theconduction band energy level of metal oxide structure and when a solarcell is produced, the open voltage decreases to reduce the photoelectricconversion efficiency.

In other mixing methods, the conditions are also preferably adjusted togive an energy sufficiently high to cause a mechanochemical reaction inthe particle groups mixed. For example, the mechanochemical reaction maybe allowed to proceed by adjusting the rotation number, residence timeor the like in the case of a high-speed rotary grinder; by adjusting thestirring rate, mass of medium, stirring time or the like in the case ofa stirring mill; and by adjusting the pressure of carrier gas, residencetime or the like in the case of a jet grinder.

For detecting the mechanochemical reaction, a method of measuring thechange of optical band gap (hereinafter referred to as BG) betweenbefore and after dry mixing is known.

The change in BG of metal oxide is considered to occur because themolecular orbital in the vicinity of metal oxide particle surface ischanged by the mechanochemical reaction. The particles differing in theprimary particle size differ also in the lattice state on the particlesurface and therefore, difference is present in the BG of theseparticles. When particles differing in BG are combined with each otherby the mechanochemical reaction, a new molecular orbital is generated togive a value different from BG before the mechanochemical reaction.Also, a phenomenon that crystallinity on the particle surface decreasesand BG changes can occur. Therefore, by measuring the difference of BGbetween before and after dry mixing (hereinafter simply referred to asABG), the bonding or surface state of a particle of Particle Group A anda particle of Particle Group B can be specified. The method formeasuring BG and ABG is described below.

The relationship between the wavelength and the absorbance is measured,for example, by using an integrating sphere-type spectrophotometer ModelUV-2400 or Model ISR-240A manufactured by Shimadzu Corporation. Atangent line is drawn with respect to inflection point of the obtainedabsorbance pattern (see, FIG. 3) and the point (absorption edgewavelength) where the tangent line and the wavelength axis are crossedis read. FIG. 3 shows one example of the relationship between theabsorbance pattern and the absorption edge wavelength.

BG is represented by the following formula:E=1240/λA(wherein E represents BG [eV] and λ represents the absorption edgewavelength [nm]) and therefore, assuming that BG and absorption edgewavelength before dry mixing are BGO [eV] and λ0 [nm], respectively, andBG and absorption edge wavelength after dry mixing are BG1 [eV] and λ1[nm], respectively, the BG values before and after dry mixing are:BG0=1240/λ0, andBG1=1240/λ1respectively. Accordingly, ΔBG [eV] between before and after dry mixingis represented by the following formula:ΔBG=BG0-BG1=(1240/λ0)−(1240/λ1)

In general, the anatase-type titanium oxide is known to have BG of 3.2eV (see, Non-Patent Document 5). However, the BG is liable to decreaseby the mechanochemical reaction.

Other metal oxides or a mixture thereof show the same BG decreasingtendency.

In the production method of the present invention, ABG between beforeand after dry mixing of the obtained metal oxide structure is preferablyfrom 0.01 to 0.45 eV. If ΔBG is less than 0.01 eV, this reveals lessbonding of particles with each other and the electron transfer amongparticles is difficult to occurs. If ΔBG exceeds 0.45 eV, thecrystallinity on the particle surface greatly decreases and this causesnot only reduction in the electron transfer speed but also seriousdecrease in the conduction band energy level of metal oxide structure,as a result, when a solar cell is produced, the open voltage lowers todecrease the photoelectric conversion efficiency.

The titanium oxide structure of the present invention is characterizedin that BG1 is from 2.7 to 3.1 eV.

The metal oxide structure obtained by the production method of thepresent invention can also be used as a dispersion of titanium oxidestructure by dispersing it in a solvent capable of dispersing the metaloxide, such as water, ethanol, acetone, acetonitrile, ethylene carbonateand propylene, or in a mixed solvent thereof. To the dispersing element,a binder may be added, such as one or a mixture of polymer compoundsselected from polyethylene glycol, polyvinyl alcohol, poly-N-vinylacetamide, polyacrylate, an N-vinylacetamide-sodium acrylate copolymer,an N-vinylacetamide-acrylamide copolymer, polyacrylamide, anacrylamide-sodium acrylate copolymer, poly-N-vinylformamide,polytetrafluoroethylene, a tetrafluoroethylene-polypropylene fluoridecopolymer, a tetrafluoroethylene-polyfluoroalkyl vinyl ether copolymer,polyvinyl fluoride, polyvinylidene fluoride, a styrene-butadienecopolymer, polyvinylpyridine, a vinylpyridine-methyl methacrylatecopolymer and polyvinylpyrrolidone. The binder as used herein means amaterial having an activity of preventing cracking from occurring at thetime of coating the dispersing element on a substrate or the like andforming a film, or preventing stripping from the substrate. Among thesebinders, preferred are polyethylene glycol, polyvinyl alcohol,poly-N-vinylacetamide, polyacrylamide, polyacrylate, anN-vinylacetamide-sodium acrylate copolymer, an acrylamide-sodiumacrylate copolymer and polytetrafluoroethylene. In the case of using apolyacrylate, preferred examples of the salt include alkali metals andalkaline earth metals. Among these salts, sodium, lithium, potassium,ammonium and magnesium are more preferred.

As the molecular weight of the binder is higher, the performance is moreenhanced. Specifically, the average molecular weight is preferably 500or more, more preferably 10,000 or more.

In the method for producing a metal oxide structure of the presentinvention, assuming that the energy constant at the time of adding adispersion medium to the obtained metal oxide structure and wet-mixingthe structure by a ball mill is k2, the relationship with the energyconstant k1 at dry mixing is preferably k2 a k1, more preferably8.0×k1≧k2≧1.5×k1, and most preferably 5.0×k1≧k2≧2.5×k1.

In the metal oxide structure produced by dry mixing, the metal oxidestructure are also aggregated with each other and the void portion isreduced. As long as the tap density of the metal oxide structureobtained by dry mixing is 0.45 g/cm³ or less, when a solar cell isproduced, the diffusibility of electrolytic solution is not greatlyaffected. However, if the tap density exceeds 0.45 g/cm³, thediffusibility of electrolytic solution sometimes decreases to causereduction in the performance of the solar cell. In order to avoid thereduction in the performance of the solar cell, the aggregate of metaloxide structure with each other is preferably dispersed by wet mixing.The wet mixing method is not particularly limited insofar as it mixesthe metal oxide with a dispersion medium and disaggregates the aggregateof metal oxide structure. For example, a ball mill, a high-speed rotarygrinder or a stirring mill may be used. In the case of wet mixing by aball mill, as the energy constant k2 increases, the dispersibility isenhanced. However, experimentally, it is effective to adjust the k2 to1.0 times or more the energy constant k1 at dry mixing. Also, althoughthe dispersing effect is higher as the k2 is larger, the upper limit ofk2 is determined from the economical viewpoint.

If the tap density of the metal oxide structure after dry mixing exceeds1.0 g/cm³, the metal oxide cannot be dispersed unless the k2 at wetmixing is set to a large value, and this is economicallydisadvantageous.

The metal oxide which can be used in the production method of thepresent invention is titanium oxide or a mixture of titanium oxide andat least one metal oxide selected from zinc oxide, niobium oxide,tantalum oxide, zirconium oxide, tin oxide and tungsten oxide.

The metal oxide structure of the present invention or theabove-described metal oxide structure dispersion may also be used as anelectrode for dye solar cells by coating it on an electrode plate suchas electrically conducting glass substrate and forming a thin film.

The electrode base material constituting the electrode substrate of adye sensitized solar cell containing the metal oxide of the presentinvention may be glass or an organic polymer.

Specific examples of the organic polymer include polyolefins such aspolyethylene, polypropylene and polystyrene, polyamides such as nylon 6,nylon 66 and aramid, polyesters such as polyethylene terephthalate,polyethylene naphthalate and unsaturated polyester, polyvinyl chloride,polyvinylidene chloride, polyethylene oxide, polyethylene glycol,silicon resin, polyvinyl alcohol, vinyl acetal resin, polyacetate, ABSresin, epoxy resin, vinyl acetate resin, cellulose and cellulosederivatives (e.g., rayon), urethane resin, polyurethane resin,polycarbonate resin, urea resin, fluororesin, polyvinylidene fluoride,phenol resin, celluloid, chitin, starch sheet, acrylic resin, melamineresin and alkyd resin. Among these, polyethylene terephthalate andpolyethylene naphthalate are preferred.

The transparent electrode substrate can be obtained by forming anelectrically conducting oxide thin film such as tin oxide,fluorine-doped tin oxide, indium oxide, zinc oxide, antimony oxide or amixture thereof, on the above-described electrode base material. Amongthese, fluorine-doped tin oxide (FTO), indium tin oxide (ITO) and amixture thereof are preferred as the electrically conducting oxide thinfilm. Examples of the method for forming the thin film include a methodof spraying an ethanol solution of indium chloride and tin chloride ontothe heated electrode base material, a method of sputtering the objectiveelectrically conducting oxide target in an Ar gas atmosphere, a methodof vacuum-depositing the objective electrically conducting oxide in anoxygen atmosphere, and an ion-plating method. As a post-treatment,heating the electrode substrate in an oxidizing atmosphere at atemperature selected according to the electrode base material andthereby enhancing the crystallinity is also effective. Depending on thethin-film formation method, the surface resistance of the electrodesubstrate varies, but in any method, the thin film is preferably formedto give a surface resistivity of 20 Ω/□ or less.

The method for forming the metal oxide structure into a thin film on theelectrode substrate is divided into a step of coating a metal oxidestructure dispersion on the electrode substrate and a drying stepsubsequent thereto. Examples of the method for coating the dispersion atthe coating step include a squeegee method, a doctor blade method, ascreen printing method, a spray method and a spin coating method. Othermethods may also be used without any particular limitation as long asthe film thickness can be adjusted.

Examples of the drying method at the step of drying the dispersioninclude a method of blowing a hot air by a drier or the like to thecoated film, a method of irradiating an infrared ray, a method ofelevating the temperature of electrode, and a method of blowing a dryair to the coated film. Other than these, a method of evaporating thesolvent from the metal oxide structure dispersion solution coated on theelectrode substrate may also be used without any particular limitationinsofar as the drying temperature is a temperature of not causingdeformation or denaturing of the electrode substrate.

In the thus-obtained thin film of metal oxide structure, fine cracks orthe like may be present and when a solar cell is produced, the electrodesubstrate may come into direct contact with the electrolytic solution tocause a reverse electron transfer (leakage current). For preventingcracks, it is preferred to previously form a dense metal oxide layer onthe electrode substrate in advance of coating the metal oxide structuredispersing element (hereinafter, the previously formed dense metal oxidelayer is called an undercoat layer and the material for forming theundercoat layer is called an undercoating agent). The undercoating agentis preferably a particle having a primary particle size of 20 nm orless, more preferably a metal oxide having good dispersibility, and mostpreferably ultrafine particulate titanium oxide.

The undercoating agent may be bound to the electrode substrate, forexample, by a method of coating the undercoating agent and thereafterbaking it at 300° C. or more. However, if baked at this temperature,deformation or denaturing may occur depending on the constructionmaterial of the electrode base material. In such a case, it is effectiveto add a binding component to the undercoating agent. The bindingcomponent is a substance having a function of binding the metal oxidestructure to the electrode substrate. The binding compound may be atleast one compound selected from silica compounds, zirconia compounds,alumina compounds and titanium compounds which are soluble in water oran organic solvent. Examples thereof include oxychloride,hydroxychloride, nitrate, ammonium carbonate and propionate of variousmetals. These binding components can bind the undercoating agent to theelectrode substrate even by the drying at an ordinary temperature or arelatively low temperature.

The amount of the binding component added must be controlled not toinhibit the electron transfer and the amount added is preferably, interms of the weight ratio of metal contained in the binding component asconverted into a metal oxide, from 3 to 200 parts by weight per 100parts by weight of the metal oxide structure.

In one example of the method for obtaining the thin film, an ultrafineparticulate titanium oxide sol having a binder component is coated on anelectrically conducting substrate using polyethylene terephthalate asthe electrode base material and dried at 120° C. and then the metaloxide dispersing element obtained by the production method of thepresent invention is coated thereon by spraying and heated in a hot airdrier at 120° C. for 20 minutes.

Also, a photocatalytic film or UV-absorbing film having hightransparency may be provided on outer side surfaces (two surfaces) ofthe electrode base material.

By providing a photocatalytic film, the electrode surface can be keptclean and thereby, the incident light into the cell can be preventedfrom decreasing in aging. The photocatalyst particle constituting thephotocatalytic film is not particularly limited but ultrafineparticulate transition metal oxides are preferred and among these,ultrafine particulate titanium oxide and ultrafine particulate zincoxide are more preferred.

The photocatalytic film is described below.

(Constitution 1)

At least a photocatalytic thin film having a photocatalytic activity andat the same time, giving a light linear transmittance of 50% or more,preferably 80% or more, for light at a wavelength 550 nm is formed onthe outer surfaces of the electrode base material.

(Constitution 2)

In Constitution 1, the thickness of the photocatalytic thin film is fromabout 0.1 to about 5 μm.

In Constitution 1 or 2, a precoat thin film having light transmittingproperty may be provided between the outer surface of the electrode basematerial and the photocatalytic thin film. The thickness of the precoatthin film is preferably from about 0.02 to about 0.2 μm. Furthermore,the precoat thin film is preferably formed of a material mainlycomprising SiO₂ or a precursor thereof.

As for the production method of the photocatalytic film, aphotocatalytic thin film can be formed on the outer surfaces of theelectrode base material by pyrosol process, dipping, printing or CVD.The photocatalytic film may be formed after a cell is assembled or anelectrode base material having previously formed thereon thephotocatalytic film may be produced.

By forming at least a photocatalytic thin film having a photocatalyticactivity and at the same time, giving a light linear transmittance of50% or more, preferably 80% or more, for light at the wavelength of 550nm, the outer side surfaces of the electrode can be kept clean over along period of time, as a result, the amount of light entering into thecell can be kept large and the photoelectric conversion efficiency canbe maintained. Furthermore, when the photocatalyst particle is ultrafineparticulate titanium oxide or ultrafine particulate zinc oxide, theultraviolet light is satisfactorily cut by the photocatalytic film andtherefore, the organic materials (e.g., dye, electrolyte component)present in the cell can be prevented from aging deterioration due toultraviolet light.

As the material for forming the titanium oxide thin film, the followingultrafine particulate titanium oxide sol can be used. Examples of themethod for producing a ultrafine particulate titanium oxide sol includethe method described in Japanese Unexamined Patent Publication (Kokai)No. 11-43327. For example, the ultrafine particulate titanium oxide solcan be obtained by hydrolyzing titanium tetrachloride.

In this case, if the titanium tetrachloride concentration in an aqueoustitanium tetrachloride solution hydrolyzed is too low, the productivityis low and at the time of forming a thin film from the produced waterdispersion titanium oxide sol, the efficiency disadvantageouslydecreases. On the other hand, if the concentration is excessively high,the reaction vigorously proceeds and the obtained titanium oxideparticle is not preferred as a transparent thin film-forming materialbecause this is not a fine particle and shows poor dispersibility.Therefore, the method of producing a sol having a high titanium oxideconcentration by hydrolysis and diluting it with a large amount of waterto adjust the titanium oxide concentration to 0.05 to 10 mol/liter isnot preferred. The titanium oxide concentration is preferably adjustedto 0.05 to 10 mol/liter at the production of sol and this may beattained by setting the titanium tetrachloride concentration in anaqueous titanium tetrachloride solution hydrolyzed to a value not sodifferent from the concentration of titanium oxide produced, namely,approximately from 0.05 to 10 mol/liter. If desired, the concentrationmay be adjusted to 0.05 to 10 mol/liter by adding a slight amount ofwater or concentrating the solution in a later step.

The temperature at the hydrolysis is preferably from 50° C. to theboiling point of the aqueous titanium tetrachloride solution. If thetemperature is less than 50° C., the hydrolysis reaction takes a longtime and this is not preferred. The hydrolysis is performed by elevatingthe temperature to a predetermined temperature and holding it forapproximately from 10 minutes to 12 hours. This holding time may beshorter as the hydrolysis temperature is higher. The hydrolysis of theaqueous titanium tetrachloride solution may be performed by a method ofheating a mixed solution of titanium tetrachloride and water in areaction tank at a predetermined temperature or by a method ofpreviously heating water in a reaction tank, adding thereto titaniumtetrachloride and elevating the temperature to a predeterminedtemperature. By this hydrolysis, brookite titanium oxide where anatasetype and/or brookite type are mixed is generally obtained.

In the case of increasing the brookite titanium oxide content, a methodof previously heating water in a reaction tank at 75 to 100° C., addingthereto titanium tetrachloride and performing the hydrolysis in thetemperature range from 75° C. to the boiling point of the solution issuitably used. According to this method, brookite titanium oxide canoccupy 70 wt % or more of the entire titanium oxide produced.

The temperature elevating rate of the aqueous titanium tetrachloridesolution at the hydrolysis is preferably higher, because finer particlescan be obtained. The temperature elevating rate is preferably 0.2°C./min or more, more preferably 0.5° C./min or more. With thistemperature elevating rate, the titanium oxide particles in the sol canhave an average particle size of 0.5 μm or less, preferably from 0.01 to0.1 μm and furthermore, a particle having high crystallinity can beobtained.

The production method of the water dispersion titanium oxide sol for usein the present invention is not limited to the batch system but acontinuous system using a continuous tank for the reaction tank may alsobe employed, where while continuously charging titanium tetrachlorideand water, the reaction solution is taken out from the side opposite thecharging port and subsequently subjected to a dechlorination treatment.The produced sol is adjusted to a chloride ion concentration of 50 to10,000 ppm by subjecting it to dechlorination, addition of water withinthe range of not causing problems, dehydration or the like. Thedechlorination treatment may be performed by a generally known methodand, for example, electrodialysis, ion exchange resin or electrolysismay be used. The degree of dechlorination may be known by using the pHof sol as a measure. When the chlorine ion is from 50 to 10,000 ppm, thepH is from about 5 to 0.5, and when the chlorine ion is in its preferredrange of 100 to 4,000 ppm, the pH is from about 4 to 1. An organicsolvent may also be added to the water dispersion sol of the presentinvention to disperse titanium oxide particles in the mixture of waterand organic solvent. In the case of forming a titanium oxide thin filmfrom this water dispersion titanium oxide sol, the sol produced by thehydrolysis reaction is preferably used as it is and a method ofproducing titanium oxide powder from the sol, dispersing the powder inwater and using the resulting sol is not preferred.

Examples of the method for obtaining the thin film include a methodwhere an ultrafine particulate titanium oxide sol having a bindercomponent is coated on an electrically conducting substrate usingpolyethylene terephthalate as the electrode base material and dried at120° C. and then the metal oxide dispersing element obtained by theproduction method of the present invention is coated thereon by sprayingand heated in a hot air drier at 120° C. for 20 minutes.

As the ultrafine particulate titanium oxide sol, a sol obtained byhydrolyzing titanium tetrachloride (see, for example, Kokai No.11-43327) can be used similarly to the photocatalytic material in theouter surface side of the electrode base material.

Furthermore, solidification or quasi-solidification of electrolyte iseffective for preventing leakage of electrolyte to the outside of a cellor elution of electrode substance or for avoiding problems such aselevation of internal impedance or occurrence of internal short-circuitdue to deviation or exhaustion of electrolytic solution in a cell. Tospeak specifically, a thermopolymerizable composition comprising acombination of a thermopolymerizable compound containing a(meth)acrylate having an oxyalkylene, fluorocarbon, oxyfluorocarbonand/or carbonate group-containing moiety in the molecule with apolymerization initiator which is an organic peroxide not having abenzene ring, is heat-cured and the obtained solid electrolyte can beused as the electrolyte.

To speak more specifically, the thermopolymerizable compound whichbecomes a polymer having a crosslinked and/or side chain form structureafter polymerization preferably contains a compound having apolymerizable functional group represented by the following formula (1)and/or formula (2):

wherein R¹ and R³ each represents hydrogen or an alkyl group, R² and R⁵each represents a divalent group containing an oxyalkylene,fluorocarbon, oxyfluorocarbon and/or carbonate group, R⁴ represents adivalent group having a carbon number of 10 or less, R², R⁴ and R⁵ eachmay contain a heteroatom and may have a linear, branched or cyclicstructure, x represents 0 or an integer of 1 to 10, provided that when aplurality of polymerizable functional groups represented by formula (1)or (2) are contained within the same molecule, R¹, R², R³, R⁴, R⁵ and xeach may be the same or different among respective polymerizablefunctional groups.

The polymerization initiator which is an organic peroxide not having abenzene ring is preferably an organic peroxide represented by thefollowing formula (3):

wherein x represents an alkyl or alkoxy group which may have asubstituent, Y represents an alkyl group which may have a substituent, Xand Y each may have a linear, branched or cyclic structure, and m and neach is 0 or 1, provided that the combination of (m,n)=(0,1) isexcluded.

In the case where the construction material of the electrode basematerial is glass, the drying can be performed under a relatively hightemperature condition by using an electric furnace or the like.

The thin film of the metal oxide structure preferably has a thickness ofabout 1 to about 40 μm. If the thickness is less than about 1 μm,scattering or absorption of light in the thin film is insufficient andthe photoelectric conversion efficiency decreases, whereas if thethickness exceeds about 40 μm, the diffusion resistance of electrolyteincreases or the electron transfer distance is prolonged, therefore, theperformance is not always enhanced and moreover, the film-formingoperation becomes cumbersome.

The method for producing a dye sensitized solar cell of the presentinvention is characterized by comprising a step of preparing ParticleGroups A, B and C, a step of mixing these particles by dry mixing forspecifying the BG, and a step of mixing the dry-mixed particles by wetmixing.

In the thus-obtained dye sensitized solar cell, the BG of metal oxideelectrode can be confirmed as follows.

The metal oxide electrode of the dye sensitized solar cell is dipped in0.1 mol/L of an aqueous sodium hydroxide solution or the like tothoroughly elute the dye from the metal oxide. The metal oxide electrodefrom which the dye is eluted is washed with water and dried at 120° C.for 2 hours to obtain a sample electrode. The BG of the metal oxidesupported on this sample electrode can be determined by theabove-described BG measuring method and BG calculating formula.

In the case where the metal oxide is titanium oxide, the BG thereof canbe confirmed to be from 2.7 to 3.1 eV.

The procedure for producing a dye sensitized solar cell, which isdescribed in “Sentan Koukinouzairyo” (vol. 6, Denjikiteki Kinouzairyo 2Denchizairyo) NGT corporation, pp 439-447), can be adopted.

The dye sensitized solar cell comprising the metal oxide structure ofthe present invention can be furnished with in an article having afunction of generating light, heat, sound or the like or having a movingfunction and thereby utilized as a power source for the function in anenvironment under light irradiated from not only an electric lamp forillumination, such as sunlight, room light, fluorescent lamp andincandescent lamp, but also other various light sources.

In addition, the dye-sensitized solar cell of the present invention canbe used as a composite charging device combined with a lithium ionbattery, a chemical capacitor or an electric double layer capacitor, acomposite cooling device combined with a Peltier element, or a compositedisplay device combined with a display device such as organic EL orliquid crystal display.

The dye-sensitized solar cell of the present invention can also beproduced as a composite device with a polymer cell. The polymer cellcomprises at least an electrode of taking out the electron transferaccompanying the oxidation-reduction reaction of a compound as anelectrical energy, and an electrolytic solution or a solid or gelelectrolyte, wherein the active material for positive and negativeelectrodes constituting the electrode is a n-conjugated polymer and/orquinone-base compound containing a nitrogen atom, which can involve thebinding/elimination of a proton in the electron transfer accompanyingthe oxidation-reduction reaction, the electrolytic solution or solid orgel electrolyte contains a proton, and the proton concentration in theelectrolytic solution or solid or gel electrolyte and the operatingvoltage are controlled so that the electron transfer accompanying theoxidation-reduction reaction of the active material for positive andnegative electrodes can be performed only to involve thebinding/elimination of a proton bound or coordinated to the nitrogenatom or a proton of the produced hydroxyl group.

Particularly, when the electrode substrate of the dye-sensitized solarcell is made of a resin and said assembled elements and parts arearranged on a flexible substrate, the obtained composite element may bemade to be flexible.

Examples of the article having a power-generating function, alight-emitting function, a heat-generating function, a sound-generatingfunction or a moving function include power sources for buildingmaterial, machine, vehicle, glass product, home appliance, agriculturalmaterial, electronic equipment, tool, tableware, bath goods, washingthing, furniture, stationery, clothing, hat, shoe, umbrella, windowshade, decorative window glass, cloth product, fiber, leather product,paper product, resin product, sporting goods, bedding, container,spectacle, billboard, signboard, piping, board, pipe laying, wiring,metal fitting, illumination, signal, street light, hygiene material,automobile equipment, toy, traffic signal, road sign, ornament, outdoorproduct such as tent and cooler box, artificial flower, objet d'art andcardiac pacemaker.

Learning material sets and DIY sets may be produced by assembling partswhich constitute production steps of said dye-sensitized solar cell orits composite element.

For example, a dye sensitized solar cell of the present invention can beinstalled to a bath article or equipment so as to use as an electricpower source for a water boiling heater, a bathroom television, abathroom boiled water-circulating unit or the like.

Further, a dye sensitized solar cell of the present invention can asubstitute power source in all applications or articles in which anSi-type solar cell is used.

EXAMPLES

The titanium oxide-containing metal oxide structure of the presentinvention is described in greater detail below by referring to Examplesand Comparative Examples, however, the present invention is not limitedthereto.

<Preparation of Dye Solution>

In a mixed solvent containing 50 vol % of acetonitrile (guaranteedreagent, produced by Kanto Kagaku) and 50 vol % of ethanol (guaranteedreagent, produced by Kanto Kagaku), 3 mmol/liter of a ruthenium complexdye (Ru(dcbpy)₂(NCS)₂, produced by Kojima Chemical Reagents Inc.) wasdissolved.

<Preparation of Electrolytic Solution>

In acetonitrile, 0.1 mol/liter of lithium iodide (produced by KishidaChemical Co., Ltd., purity: 97%), 0.05 mol/liter of iodine (guaranteedreagent, produced by Kanto Kagaku) and 0.5 mol/liter oftetrabutylammonium iodide (produced by Acros Organics, purity: 98%) weredissolved.

<Measuring Method of Photoelectric Conversion Efficiency>

Using a xenon lamp (UXL-150D-S, manufactured by Ushio Inc.) as a lightsource, light of 100 mW/cm² was irradiated on a dye sensitized solarcell produced. The maximum photoelectric conversion efficiency at thistime was measured by using a potentiostat (HAB151, manufactured byHokuto Denko Corporation).

Example 1

Into a 800 cm³-volume polyethylene container (φ96×133 mm) of a ball mill(AV, manufactured by Asahi Rika Seisakusho), 1.5 g of titanium oxide(supertitania (registered trademark) F-10, produced by Showa Denko K.K.)having an average primary particle size of 150 nm obtained by avapor-phase process, 13.5 g of titanium oxide (Supertitania (registeredtrademark) F-5, produced by the same company) having an average primaryparticle size of 25 nm and 500 g of 30 zirconia balls were charged andmixed at a rotation number of 80 rpm for 1 hour to perform a mixingmechanochemical reaction. The energy constant k1 was 15,360, the tapdensity of the titanium oxide structure obtained was 0.19 g/cm³ ₁ andABG was 0.18 eV. In this titanium oxide structure, contamination due toabrasion or the like by zirconia ball was not observed.

Into a 800 cm³-volume polyethylene container (φ96×133 mm) of a ballmill, 15.0 g of the titanium oxide structure, 70 g of pure water, 10 gof ethanol and 5 g of polyethylene glycol (extra pure reagent, molecularweight: 500,000, produced by Wako Pure Chemical Industries, Ltd.) werecharged and wet-mixed at a rotation number of 80 rpm for 1 hour to givean energy constant k2 of 15,360. The obtained titanium oxide structuredispersion was coated on an electrically conducting glass substrate(produced by Asahi Glass Company, Ltd.) and then baked at 5000C for 20minutes to form a titanium oxide thin film in a thickness of 10 to 12 μmon the electrically conducting glass substrate.

This titanium oxide thin film was immersed in the dye solution at 20 to25° C. overnight, thereby adsorbing the dye to obtain a dye electrode. Aplatinum counter electrode comprising an electrically conducting glasssubstrate having supported thereon platinum and the dye electrode formedinto a 5 mm square were fixed such that respective active surfaces facedwith a spacing of 30 μm, and an electrolytic solution was pouredtherebetween to produce an open-type dye sensitizing solar cell. Thephotoelectric conversion efficiency of this solar cell was 3.1%. Themeasurement results of weighted average specific surface area ofParticle Group B, tap density, BG after dry mixing, ABG andphotoelectric conversion efficiency are shown in Table 1.

Example 2

A dye sensitized solar cell was produced in the same manner as inExample 1 except for changing the titanium oxides of Example 1 to 1.5 gof vapor-phase process titanium oxide (Supertitania (registeredtrademark) G1, produced by Showa Denko K.K.) having an average primaryparticle size of 250 nm, 6.8 g of vapor-phase process titanium oxide(Supertitania (registered trademark) F-4, produced by the same company)having an average primary particle size of 30 nm and 6.7 g ofvapor-phase process titanium oxide (Supertitania (registered trademark)F-6, produced by the same company) having an average primary particlesize of 15 nm and changing the wet-mixing time to 5 hours. Thephotoelectric conversion efficiency of this solar cell was 4.0%. Themeasurement results of weighted average specific surface area ofParticle Group B, tap density, BG after dry mixing, ABG andphotoelectric conversion efficiency are shown in Table 1.

Example 3

A dye sensitized solar cell was produced in the same manner as inExample 1 except for changing the titanium oxides of Example 1 to 3.0 gof vapor-phase process titanium oxide (Supertitania (registeredtrademark) F-10, produced by Showa Denko K.K.) having an average primaryparticle size of 150 nm, 2.0 g of vapor-phase process titanium oxide(Supertitania (registered trademark) F-5, produced by the same company)having an average primary particle size of 25 nm and 10.0 g ofvapor-phase process titanium oxide (Supertitania (registered trademark)F-6, produced by the same company) having an average primary particlesize of 15 nm. The photoelectric conversion efficiency of this solarcell was 4.2%. The measurement results of weighted average specificsurface area of Particle Group B, tap density, BG after dry mixing, ABGand photoelectric conversion efficiency are shown in Table 1.

Example 4

A dye sensitized solar cell was produced in the same manner as inExample 1 except for changing the dry mixing time and wet mixing timeeach to 10 hours. The photoelectric conversion efficiency of this solarcell was 4.4%. The measurement results of weighted average specificsurface area of Particle Group B, tap density, BG after dry mixing, ABGand photoelectric conversion efficiency are shown in Table 1.

Example 5

A dye sensitized solar cell was produced in the same manner as inExample 1 except for changing the titanium oxides of Example 1 to 1.5 gof vapor-phase process titanium oxide (Supertitania (registeredtrademark) G1, produced by Showa Denko K. X.) having an average primaryparticle size of 250 nm, 10.1 g of vapor-phase process titanium oxide(supertitania (registered trademark) F-4, produced by the same company)having an average primary particle size of 30 nm and 3.4 g ofvapor-phase process titanium oxide (Supertitania (registered trademark)F-6, produced by the same company) having an average primary particlesize of 15 nm and changing the ball mill mixing to jet mill (CP-04,manufactured by Seishin Enterprise Co., Ltd.) mixing at 20° C. and 65MPa which was performed 5 times. The photoelectric conversion efficiencyof this solar cell was 3.7%. The measurement results of weighted averagespecific surface area of Particle Group B, tap density, BG after drymixing, ABG and photoelectric conversion efficiency are shown in Table1.

Example 6

A dye sensitized solar cell was produced in the same manner as inExample 1 except for changing the titanium oxides of Example 1 to 1.0 gof titanium oxide (Supertitania (registered trademark) F-10, produced byShowa Denko K.K.) having an average primary particle size of 150 nm,13.0 g of titanium oxide (Supertitania (registered trademark) F-5,produced by the same company) having an average primary particle size of25 nm and 1.0 g of vapor-phase process zinc oxide having an averageprimary particle size of 30 nm. The photoelectric conversion efficiencyof this solar cell was 2.7%. The measurement results of weighted averagespecific surface area of Particle Group B, tap density, BG after drymixing, ABG and photoelectric conversion efficiency are shown inTable 1. TABLE 1 Examples 1 2 3 4 5 6 Weighted average 69 75 95 69 63 69specific surface area of Particle Group B [m²/g] Energy constant k1 [−]15,360 15,360 15,360 153,600 — 15,360 Tap density after dry 0.19 0.170.15 0.57 0.24 0.18 mixing [g/m³] BG after dry mixing 2.9 2.9 3.1 2.83.0 2.8 [eV] ΔBG between before and 0.18 0.13 0.07 0.36 0.06 0.18 afterdry mixing [eV] Energy constant k2 [−] 15,360 76,800 15,360 153,60015,360 15,360 Maximum photoelectric 3.1 4.2 4.2 4.4 3.7 2.7 conversionefficiency [%] (Comparative Example 1)

Into a 500 ml-volume polyethylene bag, 1.5 g of vapor-phase processtitanium oxide (Supertitania (registered trademark) F-10, produced byShowa Denko K.K.) having an average primary particle size of 150 nm and13.5 g of vapor-phase process titanium oxide (Supertitania (registeredtrademark) F-5, produced by the same company) having an average primaryparticle size of 25 nm were charged. Then, these powder particles weremixed by shaking 50 times. The tap density of the titanium oxide mixtureobtained was 0.11 g/cm³ and ABG was 0 eV.

Using this titanium oxide mixture, a dye sensitized solar cell wasproduced in the same manner as in Example 1. The photoelectricconversion efficiency of this solar cell was 2.1%. The measurementresults of weighted average specific surface area of Particle Group B,tap density, BG after dry mixing, ABG and photoelectric conversionefficiency are shown in Table 2.

Comparative Example 2

A dye sensitized solar cell was produced in the same manner as inExample 1 except for changing the dry mixing time of Example 1 to 0.1hour and the wet mixing time to 5 hours. The photoelectric conversionefficiency of this solar cell was 2.2%. The measurement results ofweighted average specific surface area of Particle Group B, tap density,BG after dry mixing, ABG and photoelectric conversion efficiency areshown in Table 2.

Comparative Example 3

A dye sensitized solar cell was produced in the same manner as inExample 1 except for changing the dry mixing time of Example 1 to 5hours and the wet mixing time to 2 hours. The photoelectric conversionefficiency of this solar cell was 2.4%. The measurement results ofweighted average specific surface area of Particle Group B, tap density,BG after dry mixing, ΔBG and photoelectric conversion efficiency areshown in Table 2.

Comparative Example 4

A dye sensitized solar cell was produced in the same manner as inExample 1 except for changing the dry mixing time to 10 hours. Thephotoelectric conversion efficiency of this solar cell was 2.3%. Themeasurement results of weighted average specific surface area ofParticle Group B, tap density, BG after dry mixing, ABG andphotoelectric conversion efficiency are shown in Table 2.

Comparative Example 5

Into a 800 cm³-volume polyethylene container (φ96×133 mm) of a ballmill, 1.5 g of titanium oxide (Supertitania (registered trademark) F-10,produced by Showa Denko K.K.) having an average primary particle size of150 nm obtained by a vapor-phase process, 13.5 g of titanium oxide(Supertitania (registered trademark) F-5, produced by the same company)having an average primary particle size of 25 nm, 70.0 g of pure water,10.0 g of ethanol, 5.0 g of polyethylene glycol (molecular weight:500,000) and 500 g of 30 zirconia ball were charged. Then, these weremixed at a rotation number of 80 rpm for 1 hour to obtain a titaniumoxide dispersion solution. A dye sensitized solar cell was produced inthe same manner as in Example 1 except for using the titanium oxidedispersion solution obtained above in place of the titanium oxidestructure dispersion. The photoelectric conversion efficiency of thissolar cell was 2.2%. The measurement results of weighted averagespecific surface of Particle Group B and photoelectric conversionefficiency are shown in Table 2. TABLE 2 Comparative Examples 1 2 3 4 5Weighted average specific 69 69 69 69 69 surface area of Particle GroupB [m²/g] Energy constant k1 [−] — 1,540 76,800 153,600 15,360 Tapdensity after dry 0.11 0.10 0.48 0.57 — mixing [(g/m³] BG after drymixing [eV] 3.2 3.2 2.9 2.8 — ΔBG between before and 0.00 0.00 0.26 0.36— after dry mixing [eV] Energy constant k2 [−] 15,360 76,800 30,72015,360 15,360 Maximum photoelectric 2.1 2.2 2.4 2.3 2.2 conversionefficiency [%] Industrial Applicability

According to the present invention, a metal oxide structure ensuring alarge adsorbed amount of sensitizing dye and smooth electron transfer,and a production method thereof are provided.

By using the metal oxide structure of the present invention, a dyesensitized solar cell having a high photoelectric conversion efficiencycan be obtained and the present invention has a practical value inindustry.

1. A titanium oxide structure having an optical band gap (hereinafterreferred to as “BG”) of 2.7 to 3.1 eV as calculated from absorbancemeasured by an integrating sphere-type spectrophotometer and having atap density of 0.15 to 0.45 g/cm³.
 2. A metal oxide structure obtainedby dry-mixing a plurality of metal oxide powder particles differing inthe particle size, wherein assuming that an optical band gap(hereinafter referred to as “BG”) of raw material metal oxide is BG0 andthe BG of metal oxide after the dry mixing is BG1, the (BG0-BG1) is from0.01 to 0.45 eV.
 3. A method for producing a metal oxide structure,comprising dry-mixing a metal oxide, wherein assuming that an opticalband gap (hereinafter referred to as “BG”) of raw material metal oxideis BG0 and the BG of metal oxide after the dry mixing is BG1, the mixingis performed to give a (BG0-BG1) of 0.01 to 0.45 eV.
 4. The method forproducing a metal oxide structure as claimed in claim 3, wherein the drymixing is at least one method selected from a ball mill, a high-speedrotary grinder, a stirring mill and a jet grinder.
 5. The method forproducing a metal oxide structure as claimed in claim 3, wherein the drymixing is performed by a ball mill and assuming that the total mass ofpowder particles mixed is wp (g), the mass of medium is wm (g), theinner diameter of ball mill container is d (m), the rotation number is n(rpm) and the mixing time is t (minute), the energy constant k1 at thedry mixing represented by the following relationship:k1=wm/wp×d×n×t is from 3,000 to 250,000.
 6. The method for producing ametal oxide structure as claimed in claim 5, wherein the energy constantk1 is from 10,000 to 1510,00.
 7. The method for producing a metal oxidestructure as claimed in claim 5, wherein the energy constant k1 is from10,000 to 50,000.
 8. The method for producing a metal oxide structure asclaimed in any one of claims 3 to 7, wherein the raw material metaloxide comprises a metal oxide powder having an average primary particlesize of 100 to 500 nm (hereinafter referred to as Particle Group A) anda metal oxide powder having an average primary particle size of 10 to 40nm (hereinafter referred to as Particle Group B), the partial sizesbeing as converted from the specific surface area determined by the BETmethod.
 9. The method for producing a metal oxide structure as claimedin claim 8, wherein Particle Group B is a mixture of a metal oxidepowder having an average primary particle size of 20 to 40 nm(hereinafter referred to as Particle Group C) and a metal oxide powderhaving an average primary particle size of 10 to 20 nm (hereinafterreferred to as Particle Group D), the particle sizes being as convertedfrom the specific surface area determined by the BET method.
 10. Themethod for producing a metal oxide structure as claimed in claim 8 or 9,wherein the average specific surface area of Particle Group B is from 60to 110 m²/g.
 11. The method for producing a metal oxide structure asclaimed in any one of claims 8 to 10, wherein at least one of ParticleGroups A to D is a metal oxide synthesized by a gas phase process. 12.The method for producing a metal oxide structure as claimed in any oneof claims 3 to 11, wherein the tap density is from 0.15 to 1.0 g/cm³.13. The method for producing a metal oxide structure as claimed in anyone of claims 3 to 12, wherein the metal oxide is titanium oxide. 14.The method for producing a metal oxide structure as claimed in any oneof claims 3 to 12, wherein the metal oxide is a mixture of titaniumoxide and at least one metal oxide selected from zinc oxide, niobiumoxide, tantalum oxide, zirconium oxide, tin oxide and tungsten oxide.15. The method for producing a metal oxide structure as claimed in claim14, wherein the content of titanium oxide contained in said metal oxidemixture is 10 mass % or more.
 16. A method for producing a metal oxidedispersion, comprising adding a dispersion medium to the titanium oxidestructure claimed in claim 1, the metal oxide structure claimed in claim2 or a metal oxide structure obtained by the production method claimedin any one of claims 3 to 15, and wet-mixing these by a ball mill,wherein assuming that the total mass of powder particles mixed is wp(g), the mass of medium is wm (g), the inner diameter of ball millcontainer is d (m), the rotation number is n (rpm) and the mixing timeis t (minute), the energy constant k2 at the wet mixing is representedby the following relationship:k2=wm/wp×d×n×t and the energy constant k1 at the dry mixing satisfy thefollowing relationship:k2≧k1.
 17. The method for producing a metal oxide dispersion as claimedin claim 16, wherein the energy constant k2 at the wet mixing and theenergy constant k1 at the dry mixing satisfy the following relationship:8.0×k1≧k2≧1.5×k1.
 18. The method for producing a metal oxide dispersionas claimed in claim 16, wherein the energy constant k2 at the wet mixingand the energy constant k1 at the dry mixing satisfy the followingrelationship:5.0×k1≧k2≧2.5×k1.
 19. A titanium oxide-containing metal oxide dispersionobtained by the production method described in any one of claims 16 to18.
 20. A composition comprising the titanium oxide structure claimed inclaim 1, the metal oxide structure claimed in claim 2, a metal oxidestructure obtained by the production method claimed in any one of claims3 to 15, or the titanium oxide-containing metal oxide dispersion claimedin claim
 19. 21. A thin film comprising the titanium oxide structureclaimed in claim 1, the metal oxide structure claimed in claim 2, ametal oxide structure obtained by the production method claimed in anyone of claims 3 to 15, or the titanium oxide-containing metal oxidedispersion claimed in claim
 19. 22. The metal oxide structure-containingthin film as claimed in claim 21, wherein the film has a thickness offrom 1 to 40 μm.
 23. A method for producing a dye sensitized solar cell,comprising including the metal oxide structure obtained by theproduction thereof claimed in any one of claims 3 to 15 as a dyesensitized electrode.
 24. A method for producing a dye sensitized solarcell, comprising including the metal oxide structure obtained by theproduction method claimed in any one of claims 3 to 15 and the metaloxide dispersion claimed in any one of claims 17 to 19 as a dyesensitized electrode.
 25. A dye sensitized solar cell produced by theproduction method claimed in claim 23 or claim
 24. 26. A dye sensitizedsolar cell equipped with a dye electrode comprising, as a constituentmember, the metal oxide structure-containing thin film claimed in claim22.
 27. A dye sensitized solar cell, wherein an optical band gap(hereinafter referred to as “BG”) of titanium oxide after removing thedye from the dye electrode is from 2.7 to 3.1 eV.
 28. An article havinga power-generating function, equipped with the dye sensitized solar cellclaimed in any one of claims 25 to
 27. 29. An article having alight-emitting function, equipped with the dye sensitized solar cellclaimed in any one of claims 25 to
 27. 30. An article having aheat-generating function, equipped with the dye sensitized solar cellclaimed in any one of claims 25 to
 27. 31. An article having asound-generating function, equipped with the dye sensitized solar cellclaimed in any one of claims 25 to
 27. 32. An article having a movingfunction, equipped with the dye sensitized solar cell claimed in any oneof claims 25 to 27.